This invention relates to a device and method for determination of the strength of a sheet of paper, and more specifically, to a device and method to determine the strength based upon monitoring the variations in the intensity of a narrow beam of light transmitted through the sheet as the sheet moves perpendicularly through the beam.
Paper is produced from a suspension of fibers. These fibers are usually made of cellulose, derived mainly from wood and rags. The evenness of the distribution of these fibers in a sheet of paper is of paramount importance to the optical and printing properties of the sheet. Therefore, one of the chief goals for a paper maker is to develop a paper making process and adjust the parameters of the process to achieve as even a "basis weight" or distribution of these fibers in the finished sheet material as possible. In the paper making art, the term "basis weight" refers to the weight of the paper-forming fibers per unit area of the sheet surface.
Among the critical characteristics of paper and other sheet materials which are important to both manufacturers and users is strength. Many different methods for measuring strength have been proposed in the past, but virtually all suffer from a great disadvantage, namely, the tests are destructive and cannot be used "on line." A number of standardized tests have been devised to provide a basis for specifications by which paper can be bought and sold, and these tests provide arbitrary but nevertheless useful strength indices for comparing the strengths of various papers. Unfortunately, all are destructive, and none can be used "on line." The more common tests are a standardized tensile test, the so-called "STFI" compression test, and the "burst pressure" or "Mullen" test.
In the standard tensile test, a strip of paper is held between two clamps and loaded in tension at a predetermined rate. The loading at failure is taken to be a measure of the tensile strength of the paper. There are a number of standardized procedures which have been adopted to perform this test, e.g., TAPPI Standard T404os-76 and ASTM Standard D828.
The "STFI" compression test for heavy papers is a standardized test whose procedure has been established by the Swedish Technical Forest Institute, as specified by the identifiers: Scan P46 Column 83. In this test a strip of paper to be tested is held between a pair of clamps which are moved together at a fixed rate while the compressive force is monitored. "Rupture" occurs when the compressive force passes a peak and begins to drop. The force at "rupture" is taken as the compressive strength of the paper. Other standard specifications for this test are, e.g., TAPPI 7818os-76 and ASTM D1164.
The strengths of papers as measured by the foregoing tests typically have different values depending on whether the test strip is cut in the machine direction or the cross direction.
A "Mullen" or burst pressure test is conducted by clamping a sample of the paper between two circular clamping rings having a specified standard inside diameter, and building up pressure on one side of the paper until the paper bursts (using a rubber diaphragm and liquid pressure). The pressure required to burst the paper is known as the "burst pressure" and is the figure often used to specify the required strength. Common burst pressure specifications are TAPPI 403os-76 and ASTM D774.
Needless to say, none of these tests lend themselves to use in connection with the continuous measurement of paper strength. Because of their widespread popularity, however, it is desirable that any method used to measure the strength of paper provides results which correlate with one of the recognized standard tests.
In addition to strength, another important paper parameter is the "formation" of the sheet. There is, apparently, no standard definition of "formation." However, for the present purpose "formation" will be defined as the manner in which fibers forming a paper sheet are distributed, disposed and intermixed within the sheet. In all paper sheets, the sheet-forming fibers are, at least to a certain extent, unevenly distributed in bunches called "flocs." However, sheets of paper having generally evenly distributed, intertwined fibers are said to have good formation. Conversely, when the fibers forming the sheet are unacceptably unevenly distributed in flocs, the paper sheet is grainy rather than uniform and is said to have poor formation.
Some researchers have found a correlation between paper formation and strength. However, their research has been limited and primarily theoretical and has not resulted in a device which can be used in practical applications to measure strength of paper being produced on a paper machine. Moreover, although a variety of devices have been tested for measuring paper formation, as will be discussed hereinafter, many are incapable of accurately measuring formation, and none are capable of measuring paper strength.
In one device for measuring formation, called a basis weight sensor (or microdensitometer), a beam of light is transmitted through the sheet as the sheet passes perpendicularly through the beam. The intensity of the beam is measured by a light detector after the beam is transmitted through the paper sheet. This light detector is positioned on the opposite side of the sheet from the light source. The light detector produces an electrical signal indicative of the intensity of the transmitted beam. As the basis weight of the portion of sheet through which the light beam is passing increases, the intensity of the beam transmitted through the sheet decreases. Thus, the electrical signal from the light detector is indicative of the basis weight of the sheet.
As previously mentioned, the fibers forming every sheet of paper tend to congregate in flocs. In any one sheet, these flocs will have a variety of sizes. Thus, as the paper moves perpendicularly through the light beam, the electrical signal produced by the light detector will be modulated at a plurality of frequencies corresponding to the distribution of floc sizes and also to the speed with which the paper sheet moves through the light beam. As the sheet speed increases, the frequency with which the flocs modulate the electrical basis weight signal increases. Similarly, smaller flocs modulate the signal at higher frequencies than larger flocs. The amplitude of these modulations corresponds to the local variations in basis weight or, what amounts to the same thing, the local variations in the distribution of the fibers forming the flocs.
In one technique, the formation characterizing device displays the average peak-to-peak variation in the electrical signal produced by a basis weight sensor. The average peak-to-peak value of the electrical signal is said to indicate the magnitude of variations in the basis weight of the sheet. However, this technique has not been applied to measure strength, and for the reasons discussed below, the technique may give a false indication of the sheet formation.
In many instances, the paper maker will want to make a sheet having as even a fiber distribution as possible, i.e. one having good formation. To accomplish this, the paper maker will want to know, not only the magnitude of the variations in basis weight, but also the size distribution of the flocs. The paper maker will also want to know the strength of the sheet. However, the previously described technique, which yields only the average peak-to-peak value of the basis weight signal, gives no indication of the size of the flocs creating these variations in the basis weight signal or the strength of the sheet.
In another technique for characterizing sheet formation, a beta radiograph is made of a sample sheet of paper. Light is then passed through or reflected off of the radiograph. Variations in the intensity of a narrow beam of this light are converted into an electrical signal as the radiograph moves, at a uniform speed, perpendicularly with respect to the beam. A graphical display is produced of the amplitude of the modulations of this electrical signal as a function of the wavelengths comprising the signal. This display is called a wavelength power spectrum. FIG. 1 illustrates one such display for several grades of paper having good, intermediate and poor formation. This technique has been discussed in great detail by Norman and Wahren in a number papers, including their symposium paper "Mass Distribution and Sheet Properties of Paper."
For some commercial paper manufacturing situations, the Norman and Wahren technique may be inappropriate to measure formation. As illustrated in FIG. 1, at wavelengths below about one millimeter, there is little difference between the wavelength power spectra of a well-formed sheet and a poorly-formed sheet. However, from wavelengths of about one millimeter to thirty-two millimeters, significant differences exist. Thus, the Norman and Wahren technique produces more information than may be necessary for the paper maker to determine formation of a sheet. Another possible disadvantage of this technique is that it provides so much information that its interpretation may be difficult for the non-expert. In many commercial manufacturing situations, the paper maker may prefer a device and technique which provides him or her with only a few numbers, which together completely characterize the formation of the sheet, rather than an entire spectral display. Moreover, this technique, like the previously described technique for measuring the average peak-to-peak value of a basis weight signal, fails to provide the paper maker with an indication of the strength of the sheet.